Integrated photonic-mirror test circuit

ABSTRACT

A reflectivity test circuit is described. The reflectivity test circuit includes a symmetric structure that cancels errors in the reflectivity measurements. In particular, the reflectivity test circuit includes an optical waveguide that is optically coupled to two optical ports and two optical couplers. The optical couplers are optically coupled to adjacent optical waveguides, at least one of which is optically coupled to a third optical port and the mirror. Moreover, a length of the optical waveguide is chosen to match the round-trip optical path length in at least the one of the adjacent optical waveguides. During operation, control logic determines the reflectivity of the mirror based at least on a ratio of an optical power measured on one of the two optical ports to an input optical power on the third optical port.

GOVERNMENT LICENSE RIGHTS

This invention was made with U.S. Government support under Agreement No.HR0011-08-9-0001 awarded by DARPA. The U.S. Government has certainrights in the invention.

BACKGROUND

Field

The present disclosure relates to techniques for testing opticalcomponents. More specifically, the present disclosure relates to anintegrated test circuit for determining reflectivity of a mirror.

Related Art

Silicon photonics is a promising technology that can provide largecommunication bandwidth, large density, low latency and low powerconsumption for inter-chip and intra-chip connections. In the last fewyears, significant progress has been made in developing low-costcomponents for use in inter-chip and intra-chip silicon-photonicconnections, including: high-bandwidth efficient silicon modulators,low-loss optical waveguides, wavelength-division-multiplexing (WDM)components, high-speed CMOS optical-waveguide photo-detectors andoptical sources (such as lasers).

Many of these components, such as lasers, include an integrated mirror.However, these mirrors are often very sensitive to fabrication variationthat can adversely affect performance of the components. It is oftendifficult to accurately measure the reflectivity of integrated mirrors.Moreover, in the absence of accurate reflectivity measurements,parasitic reflective elements in photonic integrated circuits cannot beappropriately quantified without accurate characterization of theirreflectivity, and therefore cannot be reduced by iterative design,fabrication, and validation cycles.

Hence, what is needed is a test structure for measuring the reflectivityof integrated mirrors without the above-described problems.

SUMMARY

One embodiment of the present disclosure provides an integrated circuitthat includes a reflectivity test circuit. The reflectivity test circuitincludes: a first mirror; a first optical port that, during operation,selectively receives first input optical power; and a first opticalwaveguide having a first end optically coupled to the first mirror and asecond end optically coupled to the first optical port. Moreover, thereflectivity test circuit includes: a first optical coupler; a firstphotodetector that, during operation, measures a first optical power; asecond photodetector that, during operation, measures a second opticalpower; and a second optical waveguide optically coupled to the firstoptical waveguide by the first optical coupler, where a first end of thesecond optical waveguide is optically coupled to the first photodetectorand a second end of the second optical waveguide is optically coupled tothe second photodetector. Furthermore, the reflectivity test circuitincludes control logic electrically coupled to the first photodetectorand the second photodetector. During operation, the control logicdetermines a reflectivity of the first mirror based on a first ratio ofthe first optical power and the second optical power when the firstinput optical power is received on the first optical port.

Note that the first optical port may include a vertical grating coupler.

In some embodiments, the integrated circuit includes: a substrate; aburied-oxide layer disposed on the substrate; and a semiconductor layerdisposed on the buried-oxide layer. The first optical waveguide, thefirst optical coupler and the second optical waveguide may, at least inpart, be included in the semiconductor layer.

Moreover, the first optical waveguide may have a first length equal to Land the second optical waveguide has a second length equal to 2·L.

Furthermore, during operation, the control logic may determine thereflectivity at different wavelengths.

Additionally, the first mirror may include a distributed Braggreflector.

In some embodiments, the reflectivity test circuit includes: a secondoptical coupler; a first optical waveguide termination; a second opticalwaveguide termination; and a third optical waveguide optically coupledto the second optical waveguide by the second optical coupler. A firstend of the third optical waveguide may be optically coupled to the firstoptical waveguide termination and a second end of the third opticalwaveguide may be optically coupled to the second optical waveguidetermination.

Furthermore, the reflectivity test circuit may include: a second opticalcoupler; a second mirror; a second optical port that, during operation,selectively receives second input optical power; and a third opticalwaveguide optically coupled to the second optical waveguide by thesecond optical coupler. A first end of the third optical waveguide maybe optically coupled to the second mirror and a second end of the thirdoptical waveguide may be optically coupled to the second optical port.During operation, the control logic may determine the reflectivity ofthe first mirror based on a product of the first ratio and a secondratio of the second optical power and the first optical power when thesecond input optical power is received on the second optical port.

Note that the second optical port may include a vertical gratingcoupler. Moreover, the third optical waveguide may have a third lengthequal to L and/or the second mirror may include a distributed Braggreflector.

Another embodiment provides an integrated circuit that includes areflectivity test circuit. The reflectivity test circuit includes: afirst mirror; a first photodetector that, during operation, measures afirst optical power; and a first optical waveguide having a first endoptically coupled to the first mirror and a second end optically coupledto the first photodetector. Moreover, the reflectivity test circuitincludes: a first optical coupler; a first optical port that, duringoperation, selectively receives first input optical power; a secondoptical port that, during operation, selectively receives second inputoptical power; and a second optical waveguide optically coupled to thefirst optical waveguide by the first optical coupler, where a first endof the second optical waveguide is optically coupled to the firstoptical port and a second end of the second optical waveguide isoptically coupled to the second optical port. Furthermore, thereflectivity test circuit includes: a second optical coupler; a secondmirror; a second photodetector that, during operation, measures a secondoptical power; and a third optical waveguide optically coupled to thesecond optical waveguide by the second optical coupler, where a firstend of the third optical waveguide is optically coupled to the secondmirror and a second end of the third optical waveguide is opticallycoupled to the second photodetector. Additionally, the reflectivity testcircuit includes control logic electrically coupled to the firstphotodetector and the second photodetector. During operation, thecontrol logic determines a reflectivity of the first mirror based on afirst ratio of the first optical power and the second optical power whenthe first input optical power is received on the first optical port.

In some embodiments, the control logic determines the reflectivity ofthe first mirror based on a product of the first ratio and a secondratio of the second optical power and the first optical power when thesecond input optical power is received on the second optical port.

Another embodiment provides a system that includes the integratedcircuit.

Another embodiment provides a method for determining the reflectivity ofthe first mirror using one of the embodiments of the reflectivity testcircuit.

This Summary is provided merely for purposes of illustrating someexemplary embodiments, so as to provide a basic understanding of someaspects of the subject matter described herein. Accordingly, it will beappreciated that the above-described features are merely examples andshould not be construed to narrow the scope or spirit of the subjectmatter described herein in any way. Other features, aspects, andadvantages of the subject matter described herein will become apparentfrom the following Detailed Description, Figures, and Claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a drawing illustrating an existing technique for measuring thereflectivity of a mirror.

FIG. 2 is a drawing illustrating an existing technique for measuring thereflectivity of a mirror.

FIG. 3 is a block diagram illustrating a reflectivity test circuit inaccordance with an embodiment of the present disclosure.

FIG. 4 is a block diagram illustrating a reflectivity test circuit inaccordance with an embodiment of the present disclosure.

FIG. 5 is a block diagram illustrating a reflectivity test circuit inaccordance with an embodiment of the present disclosure.

FIG. 6 is a block diagram illustrating a reflectivity test circuit inaccordance with an embodiment of the present disclosure.

FIG. 7 is a block diagram illustrating an integrated circuit inaccordance with an embodiment of the present disclosure.

FIG. 8 is a block diagram illustrating a system that includes theintegrated circuit of FIG. 7 in accordance with an embodiment of thepresent disclosure.

FIG. 9 is a flow chart illustrating a method for determining thereflectivity of a first mirror in accordance with an embodiment of thepresent disclosure.

FIG. 10 is a flow chart illustrating a method for determining thereflectivity of a first mirror in accordance with an embodiment of thepresent disclosure.

Note that like reference numerals refer to corresponding partsthroughout the drawings. Moreover, multiple instances of the same partare designated by a common prefix separated from an instance number by adash.

DETAILED DESCRIPTION

Embodiments of a reflectivity test circuit, an integrated circuit thatincludes the reflectivity test circuit and a technique for determiningthe reflectivity of a mirror are described. The reflectivity testcircuit includes a symmetric structure that cancels errors in thereflectivity measurements. In particular, the reflectivity test circuitincludes an optical waveguide that is optically coupled to two opticalports and two optical couplers. The optical couplers are opticallycoupled to adjacent optical waveguides, at least one of which isoptically coupled to a third optical port and the mirror. Moreover, alength of the optical waveguide is chosen to match the round-tripoptical path length in at least the one of the adjacent opticalwaveguides. During operation, control logic determines the reflectivityof the mirror based at least on a ratio of an optical power measured onone of the two optical ports to an input optical power on the thirdoptical port.

By correcting for errors in the reflectivity measurements, this opticalcharacterization technique may significantly reduce the error and thusthe accuracy of the reflectivity measurements for the mirror. Thisimproved characterization may allow mirrors (such as laser mirrors) tobe validated prior to packaging, and may allow systematic improvementsin the design and fabrication of mirrors. Therefore, the opticalcharacterization technique may improve the performance of the mirrorsand systems that include the mirrors. In turn, the improved performancemay increase the yield and reduce the cost of the mirrors and thesystems. Consequently, the optical characterization technique mayfacilitate high-speed inter- and intra-chip silicon-photonicinterconnects, as well as associated systems (such as high-performancecomputing systems).

In the discussion that follows, mirrors (and, in particular, distributedBragg reflectors) are used as an illustration of silicon-photonicoptical components that can be characterized using the reflectivity testcircuit. However, the optical characterization technique may be used tomeasure the reflectivity of a wide variety of silicon-photonic opticalcomponents.

We now describe embodiments of the reflectivity test circuit. FIG. 1presents a drawing illustrating an existing technique for measuring thereflectivity of a mirror. This existing technique, which is based on acoupled optical waveguide, is often prone to error because it relies onknowledge of the input optical power as well as the individual lossfactors from the test setup, the optical coupler, and the opticalwaveguide that separates the optical coupler and the reflective element(such as a reflector or a mirror).

Moreover, as shown in FIG. 2, which presents a drawing illustrating anexisting technique for measuring the reflectivity of a mirror, anoptical cavity that includes two reflectors or mirrors may be used todetermine the mirror reflectivity if the optical waveguide loss isknown. However, this existing technique may only be valid for broad-bandreflectors because it assumes a constant reflectivity within a fullfree-spectral range of the optical cavity. Furthermore, the existingtechnique illustrated in FIG. 2 typically requires that transmissionthrough the reflectors is possible, which is not the case for manyhigh-reflectivity mirror designs.

These challenges are addressed by the reflectivity test circuit. FIG. 3presents a block diagram illustrating a reflectivity test circuit 300.This reflectivity test circuit includes: a mirror 310-1 (and, moregenerally, an optical component having a non-zero reflectivity); anoptical port 312-1 that, during operation of reflectivity test circuit300, selectively receives input optical power 314; and an opticalwaveguide 316-1 having an end 318-1 optically coupled to mirror 310-1and an end 318-2 optically coupled to optical port 312-1. Moreover,reflectivity test circuit 300 includes: an optical coupler 320-1;optical ports 312-2 and 312-3; an optical waveguide 316-2 opticallycoupled to optical waveguide 316-1 by optical coupler 320-1, whereoptical waveguide 316-2 has an end 328-1 optically coupled to opticalport 312-2 and an end 328-2 optically coupled to optical port 312-3.Furthermore, reflectivity test circuit 300 includes an optical coupler320-2; an optional mirror 310-2; an optional optical port 312-4 that,during operation of reflectivity test circuit 300, may selectivelyreceive input optical power 336; an optical waveguide 316-3 opticallycoupled to optical waveguide 316-2 by optical coupler 320-2, whereoptical waveguide 316-3 has an end 334-1 optically coupled to optionalmirror 310-2 and an end 334-2 optically coupled to optional optical port312-4.

Additionally, reflectivity test circuit 300 includes control logic 330that may be electrically coupled to photodetectors (not shown). Duringoperation of reflectivity test circuit 300, an optical power 324 may bemeasured using a photodetector (such as photodetector 322-1 in FIG. 4)at optical port 312-2 when input optical power 314 is received atoptical port 312-1, and an optical power 326 may be measured using aphotodetector (such as photodetector 322-2 in FIG. 4) at optical port312-3 when input optical power 336 is received at optional optical port312-4. Then, control logic 330 may determine a reflectivity of mirror310-1 based on a first ratio of optical power 324 and optical power 326when input optical power 314 is received on optical port 312-1.Alternatively, control logic 330 may determine a reflectivity ofoptional mirror 310-2 based on a second ratio of optical power 326 andoptical power 324 when input optical power 336 is received on optionaloptical port 312-4. In some embodiments, control logic 330 determinesthe reflectivity of mirror 310-1 or mirror 310-2 based on a product ofthe first ratio and the second ratio. Note that control logic 330 maydetermine the reflectivity at different wavelengths, e.g., usingmeasurements at input optical power 314 or 336 at the differentwavelengths.

Moreover, optical waveguides 316-1 and/or 316-3 may have a length equalto L and optical waveguide 316-2 may have a second length equal to 2·L.In an exemplary embodiment, one or more of mirrors 310 include adistributed Bragg reflector, one or more of optical couplers 320 includea directional coupler, and/or one or more of optical ports 312 include avertical grating coupler (which may enable wafer-scale testing withoutintegrated lasers or another optical source(s), or without integratedphotodetectors).

By using a fully symmetric reflective test circuit, errors in thereflectivity measurement may be cancelled out. In particular, referringback to FIG. 3, reflectivity test circuit 300 may include two mirrors310 (or reflective elements) integrated with two optical couplers 320that are optically coupled by optical waveguide 316-2 having a length of2·L, which is chosen to match the round-trip path length of opticalwaveguides 316-1 and 316-3 (which have an identical design) opticallycoupling optical couplers 320 and mirrors 310.

During the optical characterization technique, the reflectivity testcircuit is used to determine the product of two measurements of ratiosof optical power on optical ports 312-2 and 312-3 when a tunable opticalsignal is input into optical port 312-1 and then optical port 312-4. Inparticular, when input optical power 314 is received on optical port312-1, optical powers 324 (P₂′) and 326 (P₃′) may be measured usingphotodetectors. Then, when input optical power 336 is received onoptical port 312-4, optical powers 324 (P₂″) and 326 (P₃″) may bemeasured using photodetectors.

When the optical powers measured at each photodetector are divided, thedependence on the input optical power and any subsequent loss leading upto the optical coupler is removed. The resulting ratio includes the lossterms of optical couplers 320 to the photodetectors at optical ports312-2 and 312-3 (α_(c2), and α_(c3)), the loss terms through opticalcouplers 320 (α_(Y1), and α_(Y2)), and the loss because of crosscoupling (α_(X1), and α_(X2)). Furthermore, the additional opticalwaveguide loss between input ports and output ports is represented asα_(WG2)

₁, α_(WG4)

₃, α_(WG4)

₂ and α_(WG3)

₁. As shown below, as long as optical waveguides 316 have the same lossand mirrors 310 are the same, the reflectivity R can be determined bythe product of the two ratios. In particular,

${R = {\frac{\alpha_{c\; 2} \cdot \alpha_{Y\; 1} \cdot \alpha_{X\; 1} \cdot \alpha_{{{WG}\; 2}\leftrightarrow 1}}{\alpha_{c\; 3} \cdot \alpha_{X\; 1} \cdot \alpha_{Y\; 2} \cdot \alpha_{{{WG}\; 3}\leftrightarrow 1}} = {\frac{P_{2}^{\prime}}{P_{3}^{\prime}} = \frac{P_{2} \cdot \alpha_{c\; 2} \cdot \alpha_{Y\; 1} \cdot \alpha_{X\; 1} \cdot \alpha_{{{WG}\; 2}\leftrightarrow 1}}{P_{3} \cdot \alpha_{c\; 3} \cdot \alpha_{X\; 1} \cdot \alpha_{Y\; 2} \cdot \alpha_{{{WG}\; 3}\leftrightarrow 1}}}}},{R = {\frac{\alpha_{c\; 3} \cdot \alpha_{Y\; 2} \cdot \alpha_{X\; 2} \cdot \alpha_{{{WG}\; 4}\leftrightarrow 3}}{\alpha_{c\; 2} \cdot \alpha_{X\; 2} \cdot \alpha_{Y\; 2} \cdot \alpha_{{{WG}\; 4}\leftrightarrow 2}} = {\frac{P_{3}^{''}}{P_{2}^{''}} = \frac{P_{3} \cdot \alpha_{c\; 3} \cdot \alpha_{Y\; 2} \cdot \alpha_{X\; 2} \cdot \alpha_{{{WG}\; 4}\leftrightarrow 3}}{P_{2} \cdot \alpha_{c\; 2} \cdot \alpha_{X\; 2} \cdot \alpha_{Y\; 1} \cdot \alpha_{{{WG}\; 4}\leftrightarrow 2}}}}},{R^{2} = {\frac{\alpha_{c\; 2} \cdot \alpha_{Y\; 1} \cdot \alpha_{X\; 1} \cdot \alpha_{{{WG}\; 2}\leftrightarrow 1}}{\alpha_{c\; 3} \cdot \alpha_{X\; 1} \cdot \alpha_{Y\; 2} \cdot \alpha_{{{WG}\; 3}\leftrightarrow 1}} \cdot \frac{\alpha_{c\; 3} \cdot \alpha_{Y\; 2} \cdot \alpha_{X\; 2} \cdot \alpha_{{{WG}\; 4}\leftrightarrow 3}}{\alpha_{c\; 2} \cdot \alpha_{X\; 2} \cdot \alpha_{Y\; 1} \cdot \alpha_{{{WG}\; 4}\leftrightarrow 2}}}},{R^{2} = {\frac{\alpha_{{{WG}\; 2}\leftrightarrow 1} \cdot \alpha_{{{WG}\; 4}\leftrightarrow 3}}{\alpha_{{{WG}\; 3}\leftrightarrow 1} \cdot \alpha_{{{WG}\; 4}\leftrightarrow 2}} = {\frac{P_{2}^{\prime}}{P_{3}^{\prime}} \cdot \frac{P_{3}^{''}}{P_{2}^{''}}}}},{and}$$R = {\sqrt{\frac{P_{2}^{\prime}}{P_{3}^{\prime}} \cdot \frac{P_{3}^{''}}{P_{2}^{''}}}.}$

(Note that by taking the product of the ratios, the error terms in thereflectivity measurement cancel out.)

Alternatively (and equivalently), the reflectivity can be determinedbased on the product of ratios when the input power is applied tooptical port 312-2 and the optical powers are measured at optical ports312-1 (P₁′) and 312-4 (P₄′), and when the input power is applied tooptical port 312-3 and the optical powers are measured at optical ports312-1 (P₁″) and 312-4 (P₄″), In particular,

${R^{2} = {\frac{\alpha_{{{WG}\; 4}\leftrightarrow 3} \cdot \alpha_{{{WG}\; 1}\leftrightarrow 3}}{\alpha_{{{WG}\; 4}\leftrightarrow 2} \cdot \alpha_{{{WG}\; 2}\leftrightarrow 1}} = {\frac{P_{1}^{\prime}}{P_{4}^{\prime}} \cdot \frac{P_{4}^{''}}{P_{1\;}^{''}}}}},{and}$$R = {\sqrt{\frac{P_{1}^{\prime}}{P_{4}^{\prime}} \cdot \frac{P_{4}^{''}}{P_{1}^{''}}}.}$

Note that the reflectivity can be determined for highlywavelength-dependent mirrors such as distributed Bragg reflectors bymeasuring the optical powers as a function of the wavelength. In theseembodiments, some frequency offset between the two measurements may beneeded for very wavelength-selective structures in order to align thetwo measurements to account for subtle variations in the mirrors 310.

As described previously, optical powers may be measured usingphotodetectors. In some embodiments, the photodetectors are included inthe reflectivity test circuit. This is shown in FIG. 4, which presents ablock diagram illustrating a reflectivity test circuit 400 that includesphotodetectors 322. In some embodiments, the reflectivity test circuitincludes one or more optical sources.

In addition, as shown in FIG. 4, if optical couplers 320 are nearlyidentical and photodetectors 322 are integrated with consistent couplingloss without significant back reflection, a single ratio (such as thefirst ratio) with a single input optical power may be sufficient todetermine the reflectivity. In particular,

$R \approx {\frac{P_{2}}{P_{3}}.}$

(Thus, the reflectivity may be determined using one ratio, another ratioand/or both ratios.)

Moreover, in some embodiments non-reflective optical terminationelements are included in the reflectivity test circuit to terminateoptical waveguide(s) 316 to spurious coupled cavities that can disruptthe spectral integrity of the optical signal propagating in opticalwaveguide(s) 316. This is shown in FIG. 5, which presents a blockdiagram illustrating a reflectivity test circuit 500 that includes oneor more optical termination elements (OTE) 332. In an exemplaryembodiment, optical termination elements 332 flare light into slab modesthat are anti-reflective (such as by using an optical waveguide taperand/or a material with high absorption).

While the preceding embodiments illustrate a particular symmetricconfiguration of the reflectivity test circuit, other symmetricconfigurations may be used. This is shown in FIG. 6, which presents ablock diagram illustrating a reflectivity test circuit 600. Inparticular, reflectivity test circuit 600 may include: mirror 310-1;photodetector 322-1 that, during operation, measures optical power 610;and optical waveguide 316-1 having end 318-1 optically coupled to mirror310-1 and end 318-2 optically coupled to photodetector 322-1. Moreover,reflectivity test circuit 600 includes: optical coupler 320-1; opticalport 312-1 that, during operation, selectively receives input opticalpower 612; optical port 312-2 that, during operation, selectivelyreceives input optical power 614; and optical waveguide 316-2 opticallycoupled to optical waveguide 316-1 by optical coupler 320-1, where end328-1 of optical waveguide 316-2 is optically coupled to optical port312-1 and end 328-2 of optical waveguide 316-1 is optically coupled tooptical port 312-2.

Furthermore, reflectivity test circuit 600 includes: optical coupler320-2; optional mirror 310-2; optional photodetector 322-2 that, duringoperation, measures optical power 616; and optical waveguide 316-3optically coupled to optical waveguide 316-2 by optical coupler 320-2,where end 334-1 of optical waveguide 316-3 is optically coupled tooptional mirror 310-2 and end 334-2 of optical waveguide 316-3 isoptically coupled to optional photodetector 322-2.

Additionally, reflectivity test circuit 600 includes control logic 330electrically coupled to photodetector 322-1 and optional photodetector322-2. During operation, control logic 330 determines a reflectivity ofmirror 310-1 based on a third ratio of optical power 610 and opticalpower 616 when input optical power 612 is received on optical port312-1.

In some embodiments, control logic 330 determines the reflectivity ofmirror 310-1 based on a product of the third ratio and a fourth ratio ofoptical power 610 and optical power 616 when input optical power 614 isreceived on optical port 312-2.

The reflectivity test circuit and the optical characterization techniqueallow the reflectivity of a mirror to be measured as a function ofwavelength for wavelength-selective mirrors with high or lowreflectivity, and/or with sharp changes in the reflectivity withwavelength. Moreover, the optical characterization technique can be usedwith optical components or devices that have negligible transmission(i.e., a reflectivity near 100%) without the additional uncertaintyassociated with existing measurement techniques.

The reflectivity test circuit may be implemented on an integratedcircuit, such as a photonic integrated circuit. FIG. 7 presents a blockdiagram illustrating an integrated circuit 700. This integrated circuitincludes: a substrate 710, a buried-oxide (BOX) layer 712 disposed onsubstrate 710, and a semiconductor layer 714 disposed on buried-oxidelayer 712. Optical components in the reflectivity test circuit may, atleast in part, be included in semiconductor layer 714. In an exemplaryembodiment, substrate 710 and semiconductor layer 714 include siliconand buried-oxide layer 712 includes silicon dioxide. Thus, substrate710, buried-oxide layer 712 and semiconductor layer 714 may constitute asilicon-on-insulator technology.

Moreover, semiconductor layer 714 may have a thickness 716 that is lessthan 1 μm (such as 0.3-1 μm) or more than 3 μm. Furthermore,buried-oxide layer 712 may have a thickness 718 between 0.3 and 3 μm(such as 0.8 μm). Note that a width of the optical waveguides may be450-500 nm.

Integrated circuit 700 may be included in a system and/or an electronicdevice. This is shown in FIG. 8, which presents a block diagramillustrating a system 800. In some embodiments, system 800 includesprocessing subsystem 810 (with one or more processors) and memorysubsystem 812 (with memory).

In general, functions of the integrated circuit and system 800 may beimplemented in hardware and/or in software. Thus, system 800 may includeone or more program modules or sets of instructions stored in anoptional memory subsystem 812 (such as DRAM or another type of volatileor non-volatile computer-readable memory), which may be executed by anoptional processing subsystem 810. Note that the one or more computerprograms may constitute a computer-program mechanism. Furthermore,instructions in the various modules in optional memory subsystem 812 maybe implemented in: a high-level procedural language, an object-orientedprogramming language, and/or in an assembly or machine language. Notethat the programming language may be compiled or interpreted, e.g.,configurable or configured, to be executed by the processing subsystem.

Components in system 800 may be coupled by signal lines, links or buses.These connections may include electrical, optical, or electro-opticalcommunication of signals and/or data. Furthermore, in the precedingembodiments, some components are shown directly connected to oneanother, while others are shown connected via intermediate components.In each instance, the method of interconnection, or ‘coupling,’establishes some desired communication between two or more circuitnodes, or terminals. Such coupling may often be accomplished using anumber of circuit configurations, as will be understood by those ofskill in the art; for example, AC coupling and/or DC coupling may beused.

In some embodiments, functionality in these circuits, components anddevices may be implemented in one or more: application-specificintegrated circuits (ASICs), field-programmable gate arrays (FPGAs),and/or one or more digital signal processors (DSPs). Furthermore,functionality in the preceding embodiments may be implemented more inhardware and less in software, or less in hardware and more in software,as is known in the art. In general, system 800 may be at one location ormay be distributed over multiple, geographically dispersed locations.

System 800 may include: a VLSI circuit, a switch, a hub, a bridge, arouter, a communication system (such as awavelength-division-multiplexing communication system), a storage areanetwork, a data center, a network (such as a local area network), and/ora computer system (such as a multiple-core processor computer system).Furthermore, the computer system may include, but is not limited to: aserver (such as a multi-socket, multi-rack server), a laptop computer, acommunication device or system, a personal computer, a work station, amainframe computer, a blade, an enterprise computer, a data center, atablet computer, a supercomputer, a network-attached-storage (NAS)system, a storage-area-network (SAN) system, a media player (such as anMP3 player), an appliance, a subnotebook/netbook, a tablet computer, asmartphone, a cellular telephone, a network appliance, a set-top box, apersonal digital assistant (PDA), a toy, a controller, a digital signalprocessor, a game console, a device controller, a computational enginewithin an appliance, a consumer-electronic device, a portable computingdevice or a portable electronic device, a personal organizer, and/oranother electronic device. Note that a given computer system may be atone location or may be distributed over multiple, geographicallydispersed locations.

Moreover, the integrated circuit can be used in a wide variety ofapplications, such as: optical communications (for example, in atransceiver, an optical interconnect or an optical link), aradio-frequency filter, a bio-sensor, data storage (such as anoptical-storage device or system), medicine (such as a diagnostictechnique or surgery), a barcode scanner, and/or metrology (such asprecision measurements of distance).

Furthermore, the embodiments of the reflectivity test circuit,integrated circuit 700 and/or system 800 may include fewer components oradditional components. Although these embodiments are illustrated ashaving a number of discrete items, these optical components, integratedcircuits and the system are intended to be functional descriptions ofthe various features that may be present rather than structuralschematics of the embodiments described herein. Consequently, in theseembodiments two or more components may be combined into a singlecomponent, and/or a position of one or more components may be changed.In addition, functionality in the preceding embodiments of thereflectivity test circuit, integrated circuit 700 and/or system 800 maybe implemented more in hardware and less in software, or less inhardware and more in software, as is known in the art.

While the preceding embodiments have been illustrated with particularelements and compounds, a wide variety of materials and compositions(including stoichiometric and non-stoichiometric compositions) may beused, as is known to one of skill in the art. Thus, while a siliconoptical waveguide was illustrated in the preceding embodiments, theoptical characterization technique may be used with other materials, asis known to one of skill in the art. Furthermore, these materials andcompounds may be fabricated using a wide variety of processingtechniques, including: evaporation, sputtering, molecular-beam epitaxy,wet or dry etching (such as photolithography or direct-writelithography), polishing, etc.

We now describe embodiments of a method. FIG. 9 presents a flow chartillustrating a method 900 for determining the reflectivity of a firstmirror, which may be performed by one of the embodiments of thereflectivity test circuit. During operation, the reflectivity testcircuit receives first input optical power (operation 910) at a firstoptical port, and conveys a first portion of the first input opticalpower (operation 912) to the first mirror using a first opticalwaveguide. Then, the reflectivity test circuit optically couples asecond portion of the first input optical power (operation 914) to asecond optical waveguide using a first optical coupler, and conveys athird portion of the first input optical power (operation 916) to afirst photodetector using the second optical waveguide.

Moreover, the reflectivity test circuit measures a first optical power(operation 918) using the first photodetector, and conveys a fourthportion of the first input optical power (operation 920) to a secondphotodetector using the second optical waveguide. Furthermore, thereflectivity test circuit measures a second optical power (operation922) using the second photodetector.

Additionally, the reflectivity test circuit determines the reflectivityof the first mirror (operation 924) based on a first ratio of the firstoptical power and the second optical power, where the first opticalwaveguide has a first length equal to L, the second optical waveguidehas a second length equal to 2·L, and the third optical waveguide has athird length equal to L.

FIG. 10 presents a flow chart illustrating a method 1000 for determiningthe reflectivity of a first mirror, which may be performed by one of theembodiments of the reflectivity test circuit. During operation, thereflectivity test circuit receives first input optical power (operation1010) at a first optical port, and conveys the first input optical power(operation 1012) to a first optical coupler using a first opticalwaveguide. Then, the reflectivity test circuit optically couples a firstportion of the first input optical power (operation 1014) to the firstmirror using a second optical waveguide, and conveys a first reflectedoptical power (operation 1016) to a first photodetector using the secondoptical waveguide.

Moreover, the reflectivity test circuit conveys a second portion of thefirst input optical power (operation 1018) to a second optical couplerusing the first optical waveguide, and optically couples a third portionof the first input optical power (operation 1020) to a third opticalwaveguide. Furthermore, the reflectivity test circuit conveys the thirdportion of the first input optical power (operation 1022) to a secondmirror using the third optical waveguide, and conveys a second reflectedoptical power (operation 1024) to a second photodetector using the thirdoptical waveguide.

Additionally, the reflectivity test circuit measures a first opticalpower (operation 1026) using the first photodetector, and measures asecond optical power (operation 1026) using the second photodetector.

The reflectivity test circuit determines the reflectivity of the firstmirror (operation 1028) based on a first ratio of the first opticalpower and the second optical power, where the first optical waveguidehas a first length equal to L, the second optical waveguide has a secondlength equal to 2·L, and the third optical waveguide has a third lengthequal to L.

In some embodiments of methods 900 and/or 1000, there may be additionalor fewer operations. Moreover, the order of the operations may bechanged, and/or two or more operations may be combined into a singleoperation.

In the preceding description, we refer to ‘some embodiments.’ Note that‘some embodiments’ describes a subset of all of the possibleembodiments, but does not always specify the same subset of embodiments.

The foregoing description is intended to enable any person skilled inthe art to make and use the disclosure, and is provided in the contextof a particular application and its requirements. Moreover, theforegoing descriptions of embodiments of the present disclosure havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present disclosure tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art, and the generalprinciples defined herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentdisclosure. Additionally, the discussion of the preceding embodiments isnot intended to limit the present disclosure. Thus, the presentdisclosure is not intended to be limited to the embodiments shown, butis to be accorded the widest scope consistent with the principles andfeatures disclosed herein.

What is claimed is:
 1. An integrated circuit, comprising a reflectivitytest circuit, wherein the reflectivity test circuit includes: a firstmirror; a first optical port that, during operation, selectivelyreceives first input optical power; a first optical waveguide having afirst end optically coupled to the first mirror and a second endoptically coupled to the first optical port; a first optical coupler; afirst photodetector that, during operation, measures a first opticalpower; a second photodetector that, during operation, measures a secondoptical power; a second optical waveguide optically coupled to the firstoptical waveguide by the first optical coupler, wherein a first end ofthe second optical waveguide is optically coupled to the firstphotodetector and a second end of the second optical waveguide isoptically coupled to the second photodetector; and control logicelectrically coupled to the first photodetector and the secondphotodetector, wherein, during operation, the control logic determines areflectivity of the first mirror based on a first ratio of the firstoptical power and the second optical power when the first input opticalpower is received on the first optical port.
 2. The integrated circuitof claim 1, wherein the first optical port includes a vertical gratingcoupler.
 3. The integrated circuit of claim 1, wherein the integratedcircuit comprises: a substrate; a buried-oxide layer disposed on thesubstrate; and a semiconductor layer disposed on the buried-oxide layer;wherein the first optical waveguide, the first optical coupler and thesecond optical waveguide are, at least in part, included in thesemiconductor layer.
 4. The integrated circuit of claim 1, wherein thefirst optical waveguide has a first length equal to L and the secondoptical waveguide has a second length equal to 2·L.
 5. The integratedcircuit of claim 1, wherein, during operation, the control logicdetermines the reflectivity at different wavelengths.
 6. The integratedcircuit of claim 1, wherein the first mirror includes a distributedBragg reflector.
 7. The integrated circuit of claim 1, wherein thereflectivity test circuit includes: a second optical coupler; a firstoptical waveguide termination; a second optical waveguide termination;and a third optical waveguide optically coupled to the second opticalwaveguide by the second optical coupler, wherein a first end of thethird optical waveguide is optically coupled to the first opticalwaveguide termination and a second end of the third optical waveguide isoptically coupled to the second optical waveguide termination.
 8. Theintegrated circuit of claim 1, wherein the reflectivity test circuitincludes: a second optical coupler; a second mirror; a second opticalport that, during operation, selectively receives second input opticalpower; a third optical waveguide optically coupled to the second opticalwaveguide by the second optical coupler, wherein a first end of thethird optical waveguide is optically coupled to the second mirror and asecond end of the third optical waveguide is optically coupled to thesecond optical port; and wherein during operation, the control logicdetermines the reflectivity of the first mirror based on a product ofthe first ratio and a second ratio of the second optical power and thefirst optical power when the second input optical power is received onthe second optical port.
 9. The integrated circuit of claim 8, whereinthe second optical port includes a vertical grating coupler.
 10. Theintegrated circuit of claim 8, wherein the first optical waveguide has afirst length equal to L, the second optical waveguide has a secondlength equal to 2·L, and the third optical waveguide has a third lengthequal to L.
 11. The integrated circuit of claim 8, wherein, duringoperation, the control logic determines the reflectivity at differentwavelengths.
 12. The integrated circuit of claim 8, wherein the firstmirror and the second mirror include distributed Bragg reflectors. 13.An integrated circuit, comprising a reflectivity test circuit, whereinthe reflectivity test circuit includes: a first mirror; a firstphotodetector that, during operation, measures a first optical power; afirst optical waveguide having a first end optically coupled to thefirst mirror and a second end optically coupled to the firstphotodetector; a first optical coupler; a first optical port that,during operation, selectively receives first input optical power; asecond optical port that, during operation, selectively receives secondinput optical power; a second optical waveguide optically coupled to thefirst optical waveguide by the first optical coupler, wherein a firstend of the second optical waveguide is optically coupled to the firstoptical port and a second end of the second optical waveguide isoptically coupled to the second optical port; a second optical coupler;a second mirror; a second photodetector that, during operation, measuresa second optical power; a third optical waveguide optically coupled tothe second optical waveguide by the second optical coupler, wherein afirst end of the third optical waveguide is optically coupled to thesecond mirror and a second end of the third optical waveguide isoptically coupled to the second photodetector; and control logicelectrically coupled to the first photodetector and the secondphotodetector, wherein, during operation, the control logic determines areflectivity of the first mirror based on a first ratio of the firstoptical power and the second optical power when the first input opticalpower is received on the first optical port.
 14. The integrated circuitof claim 13, wherein the first optical port and the second optical portinclude vertical grating couplers.
 15. The integrated circuit of claim13, wherein the integrated circuit comprises: a substrate; aburied-oxide layer disposed on the substrate; and a semiconductor layerdisposed on the buried-oxide layer; wherein the first optical waveguide,the first optical coupler, the second optical waveguide, the secondoptical coupler and the third optical waveguide are, at least in part,included in the semiconductor layer.
 16. The integrated circuit of claim13, wherein the first optical waveguide has a first length equal to L,the second optical waveguide has a second length equal to 2·L, and thethird optical waveguide has a third length equal to L.
 17. Theintegrated circuit of claim 13, wherein, during operation, the controllogic determines the reflectivity at different wavelengths.
 18. Theintegrated circuit of claim 13, wherein the first mirror and the secondmirror include distributed Bragg reflectors.
 19. The integrated circuitof claim 13, wherein during operation, the control logic determines thereflectivity of the first mirror based on a product of the first ratioand a second ratio of the second optical power and the first opticalpower when the second input optical power is received on the secondoptical port.
 20. A method for determining a reflectivity of a firstmirror, the method comprising: receiving first input optical power at afirst optical port; conveying a first portion of the first input opticalpower to the first mirror using a first optical waveguide; opticallycoupling a second portion of the first input optical power to a secondoptical waveguide using a first optical coupler; conveying a thirdportion of the first input optical power to a first photodetector usingthe second optical waveguide; measuring a first optical power using thefirst photodetector; conveying a fourth portion of the first inputoptical power to a second photodetector using the second opticalwaveguide; measuring a second optical power using the secondphotodetector; and determining the reflectivity of the first mirrorbased on a first ratio of the first optical power and the second opticalpower, wherein the first optical waveguide has a first length equal toL, the second optical waveguide has a second length equal to 2·L, andthe third optical waveguide has a third length equal to L.